Known are flow measuring devices, which ascertain the flow velocity of a medium using the travel-time difference method. The use of such flow measuring devices is limited essentially to liquids or gases with low flow velocities. In the case of higher velocities, especially in the case of gases, dispersion of the ultrasonic signal can occur, so that the signal no longer completely strikes the receiver provided therefor. In given cases, the signal can completely miss the receiver. In either case, significant signal loss can be experienced.
Also known are flow measuring devices, which determine the flow velocity of a medium using thermal, mass flow measurement. This method can be applied especially for gases with high flow velocities. In the case of lower flow velocities, this measuring method is, however, often too inaccurate. Moreover, this type of measuring can only be applied for gases with known thermal conductivity coefficients. In the case of combustion exhaust gases from plants, for example, oil fields and the like, the flow velocity often cannot be exactly determined, since the composition of such gases can change.